The use of supercritical fluids as a transport medium for the manufacture of surface coatings is well known. German patent application 28 53 066 describes the use of a gas in the supercritical state as the fluid medium containing the solid or liquid coating substance in the dissolved form. In particular, the application addresses the coating of porous bodies with a protectant or a reactive or nonreactive decorative finish by immersion of the porous body in the supercritical fluid coupled with a pressure drop to effect the coating. The most significant porous bodies are porous catalysts. However, the applicant characterizes fabrics as porous bodies.
Smith, U.S. Pat. No. 4,582,731, patented Apr. 15, 1986, and U.S. Pat. No. 4,734,451, patented Mar. 29, 1988, describes forming a supercritical solution which includes a supercritical fluid solvent and a dissolved solute of a solid material and spraying the solution to produce a "molecular spray." A "molecular spray" is defined as a spray "of individual molecules (atoms) or very small clusters of the solute." The Smith patents are directed to producing fine films and powders. The films are used as surface coatings.
The aforementioned related applications are generally concerned with the formation of coatings utilizing supercritical fluids to reduce the viscosity of the coatings formulations. These applications stress the use of carbon dioxide (CO.sub.2) for generating the supercritical fluid.
U.S. patent application Ser. No. 133,068, filed Dec. 21, 1987, to Hoy, et al., discloses a process and apparatus for the liquid spray application of coatings to a substrate and minimizes the use of environmentally undesirable organic diluents. The process of the application involves:
(1) forming a liquid mixture in a closed system, said liquid mixture comprising:
(a) at least one polymeric compound capable of forming a coating on a substrate; and PA1 (b) at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture of (a) and (b) to a point suitable for spray applications; and PA1 (a) at least one polymeric component capable of forming a coating on a substrate; and PA1 (b) a solvent component containing at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture to a point suitable for spray application; and PA1 (a) at least one polymeric component capable of forming a coating on a substrate; and PA1 (b) a solvent component containing at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture to a point suitable for spray application; PA1 One problem associated with supplying a release agent to a replicating surface is the irregular nature of the deposition owing to the large amount of release agent inevitably used. Illustratively, conventional spraying of a release agent to a release surface involves propelling a solution of the release agent (generally dissolved in a solvent) by a gas under pressure. The spray comprises droplets of the release agent and the droplets coalesce on the sprayed surface to form a film of considerable thickness. If the release surface is a mold, then the release agent is sprayed into the mold onto the mold surface(s). The material to be molded is supplied to the mold and the replication of the mold surface may take place under heat and pressure. Under the operating conditions, the mold release agent is to act as a barrier that keeps the molding material from contacting the mold surface. The mold release agent does this in three ways, as a vapor, a liquid or solid. It either vaporizes and provides a vapor barrier to the surface or it liquefies without vaporization if it starts out as a solid to form a liquid barrier or loses viscosity without vaporization if it starts out as a liquid to form a liquid barrier or it is sprayed from a solvent containing solution to the mold, and the solvent evaporates in the mold to deposit a solid wax film. In almost all cases, there is a viscosity reduction in the release agent that allows it to become more uniformly coated across the mold surface. However, this does not mean that the mold release agent exists as a uniform vaporous, liquid or solid layer on the mold surface. If the amount of release agent is excessive at any place in the mold, the surface of the mold will ultimately be nonuniformly coated. The heat from the mold surface received by the material being supplied and being acted upon in the mold is not going to be uniformly applied to it and this thermal variance can adversely affect the molded object being produced. Such adverse effects will typically exist at the surface of the molded object. PA1 In the case of irregularly shaped molds or cavity molds, there is a tendency of the sprayed on release agent to pool into thicker layers owing to gravitational flow to lower surface portions in the mold. As a result, there is an assured irregularity in the temperature across the mold surface that is experienced by the material being molded. This does not mean that portions of the mold surface are devoid of mold release agent; to the contrary, the point to be made is that portions of the mold surface have too much mold release agent. PA1 For example, in the baking of goods in a baking pan, there are used release agents supplied to the surface of the baking pan that are made of vegetable oils. These oils penetrate the baking formulations such that the skin of the baked goods is essentially "french fried" by the vegetable oil and the surfaces of the baked goods have a consistency different from that of the interior of the goods. Indeed, if the condition were otherwise, one would question that the goods were properly baked. PA1 Some plastics possess crystalline and amorphous components. Penetrating release agents can attack either phase such that the surface of the molded piece is different from its interior. PA1 Many plastics that are molded are used for food applications where the food contacts the plastic. Of serious concern is the presence of mold release agents that adhere to the surface of the resulting molded plastic part that can adversely affect the performance of the molded part in one or more aspects of the use of the plastic. For example, even a thin layer of mold release agent on the surface of the plastic part has to be removed from the part or else it will contribute either a taste or texture factor to the food in contact with the part. PA1 There are many industrial applications where coating is limited to certain portions of a surface so that the uncoated surface can be subsequently used in another manner. For example, portions of a metal surface may be painted first and another portion left unpainted so that is can be used to effect bonding between surfaces. Illustrations are painted automobile or aircraft parts being adhesively bonded to other parts. In the case of solid state electronic circuits, portions of a dielectric surface are first masked before applying the electronic circuit. A problem that exists in such techniques is that the coating or masking cannot be applied in an industrial high volume production environment so that the uncoated portion occupies the minimal area on the surface for the subsequent application involved. Coating materials have a tendency to run or migrate therefore to assure that the coating-free surface remains coating-free, more of coating-free surface is allocated than is necessary for the subsequent application in which it is a functional surface. This is more of a problem where the surface is to be dip coated. It is most difficult to allocate a coating-free surface by the dipping process. It would be desirable to form coating-free portions on a surface which is to be otherwise dip coated and not have the release agent provided on the coating-free portions adversely affect the portion of the surface that is to be dip coated. PA1 It would be desirable to be able to pretreat the surface to be coated with a coating release agent at the portions of the surface that ultimately are to remain uncoated, and then apply the finish coating or masking to the whole surface, including that surface portion containing the coating release agent, and complete the coating or masking activity, such as curing or drying the coating or masking. After the activity is over, the surface where the coating release agent had been applied can be brushed to remove the unbonded portion of the coating or mask to leave a surface containing the coating release agent. PA1 The virtue of such a masking procedure resides in the ability to minimize the size of the uncoated (coating-free) portion of the surface. Such minimizes the presence of uncoated and unbonded areas on the surface. PA1 The technique would only be effective if the release agent does not migrate prior to or during the coating operation and is readily removable from the substrate. Owing in large part to the excessive amount of release agent that would be supplied to the surface by conventional techniques, migration of the release agent during some phase of the coating operation would occur. This would increase the coating-free portion of the surface in an uncontrolled manner. PA1 The technique would also require that the amount of release agent on the coating-free portion of the surface be readily removeable from the surface so as to avoid interfering with subsequent utilization of that coating-free surface. PA1 .DELTA. they fail to provide as good release properties; PA1 .DELTA. they create a water disposal problem; and PA1 .DELTA. they may adversely affect the temperature of the surface being treated. PA1 The use of organic solvents can be eliminated or minimized. PA1 The concentration of release agent on the release surface can be materially reduced. PA1 Solid release agents can be used where liquids had previously been employed because the release agent can be uniformly deposited as small particles on the release surface. PA1 The release agent can be supplied as a liquid from a sprayhead of a spray device and immediately upon clearance from the sprayhead, the spray exists as a fine mist of particles, each particle having a much greater viscosity than that of the liquid from which they are derived. PA1 The release agent may be applied to less of the surface than the other material is applied to, so that the other material covers surface containing the release agent and surface which is free of the release agent. The other material is removed from surface containing the release agent. PA1 The release agent may be applied using standard spray technology. PA1 i. at least one release agent capable of forming a thin layer or coating on the mold surface, PA1 ii. at least one supercritical fluid, and PA1 iii. optionally, a reduced amount, such as a minor amount inclusive of a small amount, of an active solvent or solvents capable of dissolving, suspending or dispersing the release agent; PA1 i. at least one wax compound capable of forming a layer on the mold surface, PA1 ii. at least one supercritical fluid, and PA1 iii. optionally, a reduced amount, such as a minor amount inclusive of a small amount, of an active solvent or solvent(s) capable of dissolving, suspending or dispersing the wax compound(s); PA1 i. at least one wax compound capable of forming a layer on the mold surface, PA1 ii. at least one supercritical fluid, and PA1 iii. optionally, a reduced amount of an active solvent or solvent(s) capable of dissolving, suspending or dispersing the wax compound(s); PA1 a. closed container means for forming a supercritical fluid, PA1 b. means for reducing the viscosity of a release agent, PA1 c. means for combining the release agent and the supercritical fluid and maintaining the supercritical fluid in the supercritical fluid state, and PA1 d. means for spraying the combination in the supercritical state to a release surface, and PA1 e. a release surface onto which the release agent is deposited.
(2) spraying said liquid mixture onto a substrate to form a liquid coating thereon.
The application is also directed to a liquid spray process in which at least one active organic solvent (c) is admixed with (a) and (b) above prior to the liquid spray application of the resulting mixture to a substrate. The preferred supercritical fluid is supercritical carbon dioxide. The process employs an apparatus in which the mixture of the components of the liquid spray mixture can be blended and sprayed onto an appropriate substrate. The apparatus contains
(1) means for supplying at least one polymeric compound capable of forming a continuous, adherent coating;
(2) means for supplying at least one active organic solvent;
(3) means for supplying supercritical carbon dioxide fluid;
(4) means for forming a liquid mixture of components supplied from (1)-(3); and
(5) means for spraying said liquid mixture onto a substrate.
The apparatus may also provide for (6) means for heating any of said components and/or said liquid mixture of components. U.S. patent application Ser. No. 133,068 demonstrates the use of supercritical fluids, such as supercritical carbon dioxide fluid, as diluents in highly viscous organic solvent borne and/or highly viscous non-aqueous dispersions coatings compositions to dilute the compositions to application viscosity required for liquid spray techniques. They further demonstrate that the method is generally applicable to all organic solvent borne coatings systems.
Copending U.S. application Ser. No. 218,910, filed Jul. 14, 1988, is directed to a liquid coatings application process and apparatus in which supercritical fluids, such as supercritical carbon dioxide fluid, are used to reduce to application consistency viscous coatings compositions to allow for their application as liquid sprays. The coatings compositions are sprayed by passing the composition under pressure through an orifice into the environment of the substrate.
In particular, the process of U.S. application Ser. No. 218,910 for liquid spray application of coatings to a substrate comprises:
(1) forming a liquid mixture in a closed system, said liquid mixture comprising:
(2) spraying said liquid mixture onto a substrate to form a liquid coating thereon by passing the mixture under pressure through an orifice into the environment of the substrate to form a liquid spray.
U.S. application Ser. No. 218,896, filed Jul. 14, 1988, is directed to a process and apparatus for coating substrates by a liquid spray in which (1) supercritical fluid, such as supercritical carbon dioxide fluid, is used as a viscosity reduction diluent for coating formulations, (2) the mixture of supercritical fluid and coating formulation is passed under pressure through an orifice into the environment of the substrate to form the liquid spray, and (3) the liquid spray is electrically charged by a high electrical voltage relative to the substrate.
In particular, the process of U.S. application Ser. No. 218,896 for electrostatic liquid spray application of coatings to a substrate comprises:
(1) forming a liquid mixture in a closed system, said liquid mixture comprising:
(2) spraying said liquid mixture onto a substrate to form a liquid coating thereon by passing the mixture under pressure through an orifice into the environment of the substrate to form a liquid spray; and
(3) electrically charging said liquid spray by a high electrical voltage relative to the substrate and electric current.
Many applications in industry utilize solid release surfaces. The function of solid release surfaces is to allow the deposition of a material onto the surface and to remove it without having the material stick to the surface. One way of forming a solid release surface is to deposit a release agent onto the surface and have the release agent replicate the surface such that any material to be deposited onto the surface is not seemingly or intended to be adversely affected by such release agent. The use of release agents on a solid surface can create considerable problems, some of which are not well appreciated. For example-
Where the release surface is a hot surface, the presence of the release agent on the surface creates a thermal gradient from the surface to the material applied to it. If the release agent is irregularly applied then the temperature across the surface of the release agent as applied to the surface will be nonuniform. This means that the material applied to the surface containing release agent will experience a variability of thermal effects. There are very few situations where this variability will not adversely affect properties of the material.
Even should the release agent be an uniformly applied layer on the release surface, the layer is relatively thick; sufficiently so that the layer penetrates the material being applied to the surface.
Even when the conventionally sprayed release agent is an uniformly applied layer on the release surface, the layer is relatively thick; sufficiently so that the thick layer precludes the use of release agents in masking portions of a surface for subsequent application of a coating to the surface.
Recognizing that release agents are high boiling or high melting materials with a high viscosity at ambient temperature and pressure conditions, in order to apply them to the release surface means that their viscosity has to be reduced at the moment they are applied to the surface. This has meant that the release agents had to be cut with solvents. Assuming that the solvents are toxicologically safe, their use introduces an environmental problem. When they are vaporized, they enter the atmosphere and are believed to contribute to smog formation. For example, hydrocarbon solvents have been widely employed as solvents for mold release agents. Enough concern exists that they represent an environmental problem because of their contribution to smog formation that water-based mold release formulations have been developed in order to eliminate these organic solvent emissions. However, the performance of these water-based compositions is significantly deficient relative to that of hydrocarbon-based materials because-
A novel system has been discovered for the application of a release agent to a surface over which another material is to be deposited and then removed. This system provides the ability to uniformly apply a release agent to a release surface and provides one or more of the following advantages: